Stocks keep notching all-time highs, but investors aren't sure if that's reason to celebrate their riches or to hunker down and prepare for the party to end.

Anyone who keeps worrying that the market's impressive run is about to peter out keeps missing out on gains. So, the pile-on continues as the market keeps slowly-but-steadily pushing to unseen heights. "Bull market psychology is controlling the day," says Michael Farr of Farr Miller & Washington. "We discount bad news and embrace all good news."

Talk about a party that never seems to quit. The Dow Jones industrial average and Standard & Poor's 500 jumped Friday to notch new all-time highs, while the Nasdaq composite set a 14-year high. The Dow closed Friday at 16,947, just 53 points or a good day away from hitting 17,000. While the day-to-day gains are relatively small, they're adding up to a powerful rally. The broad S&P 500 is up 6.2%, putting it on pace to post another good year.

• Policymakers are supporting the run. Investors took the testimony from Federal Reserve Chair Janet Yellen as an all-clear sign, Farr says. Yellen suggested last week that stocks are not particularly overvalued and indicated that the Fed is standing ready to stoke the economy if needed. That

encourages investors to own stocks, Farr says. "The Fed is the wind at our backs."

• Sitting out isn't an option. Moves by central banks around the world make it costly to not be involved in stocks, says Doug Sandler of Riverfront Investment Group. With interest rates low and an uptick inflation a possibility, holding cash is one way that appears to be a loser no matter what. "Investors recognize the penalty from sitting on the sidelines," Sandler says.

• Earnings support the rally. If stocks were rising without underlying profit power from companies, investors would get more concerned, Sandler says. Companies are expected to post nearly 7% earnings growth in the current second quarter, says S&P Capital IQ. That's more than double the earnings growth of the first quarter, and a powerful signal since corporate earnings are already at record levels. "This is still a market with a reasonable amount of upside," Sandler says.

There are worries. If the situation in Iraq keeps oil prices elevated, that could be an "economic headwind" for the market to overcome, Farr says. Though the Fed hasn't indicated it would, if it sets a deadline for "taking its foot off the gas" and sticks with it even if the economy isn't ready, stocks could suffer, Sandler says.

But for now, investors are enjoying an impressive run that some don't see winding down soon. "Everyone wants to be nervous," Sandler says. "But we can't find a reason."